The present invention relates to the creation of patterns on photosensitive optical materials, such as waveguides.like optical fibres, by placing the materials in the interference pattern generated by the intersection of at least two beams of light, preferably ultra violet light. The period of the grating or interference pattern being determined by the angle between and nature of the intersecting beams.
Referring now to FIG. 1, there is illustrated a first known set up 1 for writing gratings 2 on an optical waveguide 3 which can comprise an optical fibre. This setup 1 includes an ultra violet laser 4 which generates a UV beam 5 which is split by a beam splitter 6 into two coherent beams 7, 8. The beam splitter 6 can comprise a semi transparent mirror or a phase mask. The UV beams 7, 8 are reflected by mirrors 10,11 which reflect the coherent beams 12, 13. The two beams 12, 13 interfere with one another so as to form an interference pattern which is projected onto photosensitive optical fibre 3 so as to form grating 2.
As the photosensitive fibre 3 is only affected by the bright fringes of the interference pattern, this pattern is transformed into a permanent refractive index grating. The fibre 3 is photosensitive in that the physical properties of the fibre are modified permanently under the action of the UV light. In the case of optical fibre, it is the refractive index which is modified, but the present invention is not limited thereto and extends to other properties such as absorption, density, chemical composition or volume.
In the arrangement of FIG. 1, it is further known to add a further mirror in one of the paths of beams 7, 8 so as to invert the profile of one of the beams. This way, the two beam profiles 12, 13 will be identical at the intersection point 2, instead of one beam reversed with respect to the other when only two folding mirrors 10, 11 are used. It is further known to utilise a three mirror arrangement to scan a narrow beam along the whole of the aperture defined by the beam splitter 6 and folding mirrors 10, 11 so as to produce an extended grating 2.
Referring now to FIG. 2, there is illustrated a second known arrangement 20 for writing gratings on an optical fibre e.g. 21. In this arrangement, the optical fibre 21 is placed directly behind a phase or amplitude mask 22. A UV beam 23 is diffracted by the phase mask 22 to produce diffraction orders e.g. 24, 25. The diffraction orders 24, 25 interfere with one another to produce an interference pattern 26 directly behind the phase mask 22 which is where the photosensitive optical fibre 21 is placed. In this configuration 20, the grating period will be determined by the period of the phase mask 22. The Bragg wavelength of a grating in optical fibre 21 can only be changed by, for example, stretching the grating during writing.
As noted previously, it is possible to use the phase mask 22 as a beam splitter giving many diffraction orders including orders 24, 25. Therefore, the phase mask 22 can also be utilised as beam splitter 6 in the arrangement of FIG. 1, by altering the orientation of beam splitter 6 and UV laser source 4.
In another known arrangement, due to Kashyap, the two mirrors 10, 11 of FIG. 1 are replaced by a solid silica block such that the UV beams are totally internally reflected on the side of the block. When utilising this configuration, all orders of diffraction other than the two desired to be utilised for the interference pattern can be blocked. This configuration is also mechanically more stable, but does not allow easy tuning of the angle between the interfering beams.
In all the aforementioned interferometric configurations, whether a beam splitter or a phase mask is utilised to split the beam, the two resulting beams 7, 8 follow different physical paths before intersecting to form grating 2. Therefore, a general problem with the arrangement of FIG. 1 is that any slight mechanical perturbation in mirrors 10, 11 will change the phase of the interference pattern during the exposure generating grating 2, resulting in a loss of contrast and/or a total xe2x80x9cwashing outxe2x80x9d of the interference pattern. It has therefore been found that great care must be taken in stabilising the arrangement 1 from any vibrations, such as those caused from air currents or movement and, such a need for stabilisation severely limits the utility of the arrangement of FIG. 1.
It is therefore an object of the present invention to provide for an improved method and apparatus of writing structures such as gratings on photosensitive optical waveguides.
In a preferred aspect of the present invention, there is provided a method for creating a structure, such as a grating, in a light sensitive optical material, such as an optical fibre. The method comprises the steps of:
(1) First creating two coherent beams of light, preferably being created by means of passing a single coherent beam through a diffraction grating.
(2) Counter propagating the two beams around an optical circuit which comprises a plurality of reflective elements so that the two beams interfere at a predetermined position.
(3) Positioning the light sensitive optical waveguide at the predetermined position so as to produce the desired structure in the optical waveguide.
Although the present invention is useful for creating many different types of structures, it is most useful in creating grating structures.
The coherent beams can be created utilising a phase mask, diffraction grating or a partially transparent mirror. The optical material can comprise an optical waveguide or optical fibre. The coherent beams can be swept along the phase mask in order to create an extended structure in the photosensitive optical material. Additionally, the refractive elements can be rotated or moved to produce different structures such as chirped gratings or apodized gratings. The optical material can be placed at various angles so as to produce blazed gratings etc.
In alternative embodiments, there will be illustrated methods for creating extended grating structures, chirped gratings, apodized gratings etc. The optical material can be placed at various angles so as to produce different effects such as apodized gratings.
In accordance with a second aspect of the present invention there is provided an apparatus for creating a structure in a photosensitive material comprising:
coherent light production means for producing at least two coherent beams of light;
a plurality of reflective elements for reflecting said coherent beams along counter propagating, substantially equivalent paths so that said coherent beams interfere at a structure staging area; and
structure staging means for mounting said photosensitive material in said structure staging area.
Preferably, the coherent light production means further comprises a coherent light input means for inputting coherent light into said coherent light production means and wherein said coherent light input means can be scanned along said coherent light production means so as to produce a corresponding extended structure in said photosensitive material.